One design criterion for a circuit breaker holds that upon occurrence of a load fault which creates an unacceptably large current draw (e.g., a short circuit current) through closed contacts of a circuit breaker, the circuit breaker mechanism must open the contacts in a manner that promptly terminates the current. Certain known circuit breakers that employ one or more pivotally mounted contact arms utilize electromagnetic blow-apart, or blow-open, force to blow open the contact arm(s) upon the occurrence of such a sudden load fault. Although the blow-open force quickly initiates contact arm motion to begin tripping the circuit breaker, current may continue to arc across the contacts as the contact arm(s) swing open. consequently, further circuit breaker design principles include minimizing (and ideally eliminating) such arcing as the tripping continues. Furthermore, once current flow has terminated, any opportunity for its re-establishment must be foreclosed as the tripping concludes.
In accomplishing prompt arrest of current arcing across blowing-open contacts, it may be desirable for the circuit breaker mechanism to augment the impetus of the blow-open force as the tripping continues toward conclusion. But in doing so, the mechanism's augmentation of the force acting on the swinging contact arm(s) must not induce rebound of the contact arm(s) off of a stop to an extent that could potentially re-establish current flow.
Consider for example a circuit breaker that employs a spring-loaded, over-center toggle mechanism which goes over-center during the trip. As the mechanism goes over-center, an operating spring which had been effectively applying to the contact arm(s), a force resisting, but not preventing, the trip, now suddenly applies its force to aid the trip, driving the swinging contact arm(s) against the stop. That added force must not cause excessive contact arm rebound from the stop.
Circuit breaker design must therefore take into consideration various factors that may conflict. A better circuit breaker design will account for such factors to provide a circuit breaker that will terminate a specified fault current within a specified response time, with better assurance that current will not be re-established once the circuit breaker has been tripped. Moreover, a successful circuit breaker design should be cost and space efficient.
It is toward these and other objectives that the present invention is directed.
A known actuator for tripping a circuit breaker comprises a bi-metal strip that increasingly warps as it is increasingly heated. When heated above a certain point where a circuit breaker should be tripped, the strip will have warped sufficiently to cause the associated trip mechanism to enable the trip. A known trip mechanism conducts electric current through the length the strip inducing ohmic heating of the bi-metal.
The present invention relates to a novel construction for associating a bi-metal actuator with other associated parts that avoids conducting electric current through the entire length of the strip. The present invention is believed to cause less stress in the bi-metal and reduce the potential for damage to it, especially when a circuit breaker is tripped due to a short circuit fault.
In the illustrated embodiment, the invention is carried out by mounting a nominally flat bi-metal strip in cantilever fashion on a load terminal. More specifically, one end portion of the bi-metal is disposed flat against, and joined to, a portion of a load terminal. The other end portion of the bi-metal is spaced from the load terminal so as to be capable of warping as the thermal energy in the strip increases. When the thermal energy in the strip reaches a level where the circuit breaker should be tripped, the cantilevered portion of the strip will have warped sufficiently to enable tripping via the associated trip mechanism.
Because only the cantilevered portion of the strip warps, it may be considered as an active portion while the portion joined to the load terminal may be considered as an inactive portion because it is constrained by its joinder to the load terminal. It is believed that current passing through the load terminal passes substantially only through the inactive portion of the bi-metal so that substantially no current passes through the active portion. Because of this, it is further believed that the bi-metal should be subject to less stress and reduced potential for damage than might otherwise be the case without denigrating the effectiveness of strip warping to enable a trip. Thermal energy input to the bi-metal that is needed to operate the trip mechanism is caused by ohmic heating occurring in the inactive portion of the strip. That heat will flow from the inactive portion to the active portion because the bi-metal is a thermal conductor. Therefore the active portion of the strip will warp, but without conducting significant current, if indeed any current at all.
By further arranging the active portion of the bi-metal to face relatively closely and directly a portion of the load terminal, heat generated in that portion of the load terminal due to current flowing through it can radiate and/or convect to the confronting bi-metal, thereby augmenting the thermal energy input to the bi-metal. It is believed that this may accelerate a trip and further reduce the potential for damaging the bi-metal upon occurrence of a fault, especially one due to a short circuit.
Accordingly, one aspect of the present invention relates to an actuator for enabling a trip comprising a conductor through which current passes, a thermally responsive bi-metal having a first end portion which is disposed against a portion of the conductor and through which at least a portion of current carried by the conductor passes, the bi-metal further comprising a second end portion that is disposed in spaced relation to the conductor and is cantilever-mounted on the portion of the conductor via the bi-metal's first end portion, and that increasingly warps as thermal energy in the bi-metal increases for enabling a trip upon attaining a certain degree of warping.
Another aspect of the present invention relates to a circuit breaker comprising a first conductor member through which current passes from a line terminal to a load terminal, a mounting movably mounting the first conductor member for selective operation to a circuit-making condition enabling current to flow between the line and load terminals and to a circuit-breaking condition preventing current flow between the line and load terminals, an operating mechanism for moving the first conductor member, a trip mechanism that operates via the operating mechanism to enable the first conductor member to move from circuit-making condition to circuit-breaking condition and thereby trip the circuit breaker, and an actuator for causing the trip mechanism to enable a trip comprising a second conductor member through which current carried by the first conductor member passes, a thermally responsive bi-metal having a first end portion which is disposed against a portion of the second conductor member and through which at least a portion of current carried by the two conductor members passes, the bi-metal further comprising a second end portion that is disposed in spaced relation to the second conductor member and is cantilever-mounted on the portion of the second conductor member via the bi-metal's first end portion, that increasingly warps as thermal energy in the bi-metal increases, and that, upon attaining a certain degree of warping, causes the trip mechanism to enable a trip by enabling the first conductor member to move from circuit-making condition to circuit-breaking condition.
The foregoing, along with further features, advantages, and benefits of the invention, will be seen in the ensuing description and claims, which are accompanied by drawings. The description and drawings disclose a presently preferred embodiment of the invention according to the best mode contemplated at this time for carrying out the invention.